In the processing of thermoplastics, lubricants are added as processing aidFEDFs. On the one hand, the lubricants are capable of reducing the friction between plastic particles which makes the plastics easier to melt and promotes the formation of a homogeneous flowable melt. Lubricants acting in this way are also commonly referred to as internal lubricants.
On the other hand, lubricants used in the processing of plastics are capable of reducing the adhesion of the plastic melt to hot surfaces of machine parts or to the walls of the molds. It is assumed that the lubricants which, after their incorporation in the plastic, migrate from the plastic to the surface on account of their limited compatibility reduce adhesion. Lubricants acting in this way are also known as external lubricants or as “mold release agents”.
In principle, the use of the lubricants also has a considerable bearing on the morphology, homogeneity and surface qualities of the plastic products.
Whether an additive acts as internal or external lubricant depends on many factors, more particularly on its structure and on the nature of the plastic. In many cases, internal and external lubricating effects may even be developed alongside one another. Initial observations on lubricants in PVC and their effect as internal and external lubricants can be found in the overview in Becker/Braun, Kunststoffhandbuch Vol. 2/1, Carl Hanser Verlag, 1986, pp. 570-595.
Known lubricants for plastics include, for example, fatty acids, fatty alcohols, fatty acid esters, fatty acid complex esters, wax esters, dicarboxylic acid esters, amide waxes, metal soaps, montan waxes, hydrocarbon waxes or oxidized hydrocarbons.
Despite their ecological and economic advantages, however, natural fats and oils have never been widely used as lubricants in the production of plastics because they act too extremely as external lubricants, i.e. they barely reduce the internal lubricating effect between the plastic particles, so that a homogeneous melt flow is not obtained, and at the same time they exude and cause transparency problems because of their incompatibility.
Where such natural fats and oils have been used in the past, the plastic parts obtained had “fisheyes” and, in addition, were not transparent.
The problem addressed by the present invention was to provide lubricant combinations based on ecologically highly compatible natural fats and oils which would not have any of the known disadvantages of natural fats and oils. Rather, the invention would provide lubricant combinations which would have the positive properties of the standard lubricants used hitherto and which would still be at least partly replaced by compounds based on natural fats and oils without any significant change in those positive properties.
The problem stated above has been solved by using natural fats and oils with iodine values below 10 in admixture with typical lubricants in the lubricant combinations. Surprisingly, lubricant combinations such as these show the positive properties of the typical lubricants for plastics although they have been partly replaced by natural fats and oils with iodine values below 10.
Thus, above all, lubricant combinations of dicarboxylic acid fatty acid esters, more particularly distearyl phthalate, as more of an internal lubricant in PVC in combination with natural fats and oils having iodine values below 10, more particularly hydrogenated tallow, are comparable in their internal lubricating effect with the dicarboxylic acid fatty acid esters on their own.